Moisture Sources Research Articles

Variations in triple oxygen isotope of speleothems from the Asian monsoon region reveal moisture sources over the past 300 years

Characterization of moisture origins is crucial to understanding hydroclimatic processes. Nevertheless, documenting past atmospheric moisture contents and their sources remains challenging partially due to insufficient moisture-tracing proxies. Here, we present triple oxygen isotope compositions in 21 cave speleothems from monsoonal Asia to examine spatial moisture origin differences in eastern Asia over the past 300 years. Our data suggest an isotopic equilibrium fractionation during speleothem formation, and thus parent water 17O anomalies (Δ′17O) values reconstructed from speleothems preserve information on moisture origins and recycling across space. Notably, speleothem Δ′17O records exhibit a distinct geographical distribution, with low values in central-eastern and southern China, high values in northwestern and northern China, and relatively high values in the Yangtze River region. This spatial pattern underscores the diverse influence of different moisture sources regulated by regional hydrological circulation, and demonstrates the potential use of speleothem Δ′17O in reconstructing the moisture cycle at spatial scales.

Projected changes in atmospheric moisture transport contributions associated with climate warming in the North Atlantic

Global warming and associated changes in atmospheric circulation patterns are expected to alter the hydrological cycle, including the intensity and position of moisture sources. This study presents predicted changes for the middle and end of the 21st century under the SSP5-8.5 scenario for two important extratropical moisture sources: the North Atlantic Ocean (NATL) and Mediterranean Sea (MED). Changes over the Iberian Peninsula—considered as a strategic moisture sink for its location—are also studied in detail. By the end of the century, moisture from the NATL will increase precipitation over eastern North America in winter and autumn and on the British Isles in winter. Moisture from the MED will increase precipitation over the southern and western portions of the Mediterranean continental area. Precipitation associated with the MED moisture source will decrease mainly over eastern Europe, while that associated with the NATL will decrease over western Europe and Africa. Precipitation recycling on the Iberian Peninsula will increase in all seasons except summer for mid-century. Climate change, as simulated by CESM2 thus modifies atmospheric moisture transport, affecting regional hydrological cycles.

What causes site-specific intra-event variations of stable isotopes in precipitation in Beijing?

Stable oxygen isotope composition (δ18Op) and deuterium excess (dp) in precipitation carry important climate and hydrological information on the history of water from evaporation, transport to precipitation. This study aims to quantify the effect of potential factors on site-specific intra-event δ18Op and dp dynamics during three precipitation events in a mid-latitude region Beijing, China. The tested factors included moisture source (Ms) and upstream rainout fraction (Up) obtained by HYSPLIT backward trajectory model, radar reflectivity factor (Ze) that corresponds to raindrop size by weather radar, sub-cloud evaporation fractionation (fe) by an isotope-evaporation model, local precipitation amount (Lp) and precipitation intensity (I) by rain gauge, and relative location of the sampling sites (downwind or upwind). Intra-event δ18Op had an amplitude of 10.3‰, and 84% of the variance could be explained, in the descending order of importance, by Ms, site, Ze, fe and I. Intra-event dp had an amplitude of 16.5‰, and 57% of the variance could be explained by fe, site, Ms and I, in the descending order of importance. Absolute values of intra-event δ18Op and dp differed between two sites, 30 km apart, by up to 4.7‰ and 9.7‰, respectively. This site-specificity in δ18Op and dp was mainly caused by differences in rainout effects and I. This work provides new insights into separating the effects of various climatic and hydrological variables on high-frequency dynamics of δ18Op and dp.

Climate and atmospheric circulation during the Early and Mid‐Holocene inferred from lake‐carbonate oxygen‐isotope records from western Ireland

ABSTRACTThe Early to Mid‐Holocene experienced marked climate change over the northern hemisphere mid‐latitudes in response to changing insolation and declining ice volume. Oxygen isotopes from lake sediments provide a valuable climate proxy, encoding information regarding temperature, hydroclimate and moisture source. We present oxygen‐isotope records from two lakes in western Ireland that are strongly influenced by the North Atlantic. Excellent replication between the records suggests they reflect regional, not local, influences. Carbonate oxygen‐isotope values peaked at the start of the Holocene, between 11.2 and 11.1 cal ka bp, and then decreased markedly until 6 cal ka bp at both sites. Palaeoecological evidence supports only modest change in temperature or hydroclimate during this interval and we therefore explain the decrease primarily by a reduction in the oxygen‐isotope composition of precipitation (δ18Oppt). We show a similar decrease in δ18O values in a forward model of carbonate isotopes between 12–11 and 6–5 cal ka bp. However, the inferred reduction in δ18Oppt between the Early and Mid‐Holocene in the model is mainly linked to a decrease in the δ18O of the ocean source water from ice sheet melting whereas the lake carbonate isotope records are more consistent with changes in the transport pathway of moisture associated with atmospheric circulation change as the dominant cause.

Validating coupled flow theory for bare‐soil evaporation under different boundary conditions

AbstractEvaporation from bare soil is an important hydrological process and part of the water and energy balance of terrestrial systems. Modeling bare‐soil evaporation is challenging, mainly due to nonlinear couplings among liquid water, water vapor, and heat fluxes. Model concepts of varying complexity have been proposed for predicting evaporative water and energy fluxes. Our aim was to test a standard model of coupled water, vapor, and heat flow in the soil using data from laboratory evaporation experiments under different boundary conditions. We conducted evaporation experiments with a sand and a silt loam soil and with three different atmospheric boundary conditions: (i) wind, (ii) wind and short‐wave radiation, and (iii) wind and intermittent short‐wave radiation. The packed soil columns were closed at the bottom (no water flux) and instrumented with temperature sensors, tensiometers, and relative humidity probes. We simulated the evaporation experiments with a coupled water, vapor, and heat flow model, which solves the surface energy balance and predicts the evaporation rate. The evaporation dynamics were predicted very well, in particular the onset of stage‐two evaporation and the evaporation rates during the stage. A continuous slow decrease of the measured evaporation rate during stage‐one could not be described with a constant aerodynamic resistance. Adding established soil resistance parametrizations to the model significantly degraded model performance. The use of a boundary‐layer resistance, which takes into account the effect of point sources of moisture, improved the prediction of evaporation rates for the sandy soil, but not for the silt loam.

Spatial variability of stable isotopes in river water over the Tibetan Plateau

AbstractEstimating isotopic changes of river water on the Tibetan Plateau (TP) remains highly uncertain due to variable water sources related with seasonality of atmospheric circulations and local kinetic fractionation. The in‐depth understanding of such changes is crucial for accurate palaeoaltimetry reconstruction and for river budget evaluation which related with surviving of one‐fifth of the world's population under accelerated warming. Here, we present a comprehensive river water isotopic composition dataset (δ18O, δD and d‐excess, 1852 samples) over the TP to systematically describe spatial distribution and explore possible controlling factors of isotopic data in theses rivers. Results showed that δ18O in river water range from −20.8‰ to −5.2‰ (latitudinally, 0.92‰ per degree), enriching northwardly in the seven exorheic river basins. The d‐excess values range from −19.7‰ to 20.5‰ with the pronounced low values in endorheic rivers basins. We identified that moisture sources and atmospheric transports leave significant signals in isotopic compositions of river water in the Upper Brahmaputra (UB), Upper Mekong (UM) and Upper Yangtze (UYA) basins, and local processes modify the isotopic signals mainly in the Upper Salween (US), Upper Indus (UI), Upper Yellow (UYE) and Endorheic Rivers (ER) basins. The δ18O‐elevation relationships are only established in the UB, UI and UYE with lapse rates of −4.4‰/km, −3.4‰/km and −3.5‰/km, respectively. We suggest that as changes of moisture sources and lapse rates in different basins, it is cautious to apply δ18O‐elevation lapse rates for palaeoaltimetry reconstruction. Our dataset will benefit regional studies of hydrological, biogeochemistry, palaeoaltimetry reconstruction and palaeoclimate.

Влияние климатических и гидрологических изменений на лесорастительные условия речных пойм Донского бассейна

The article deals with the problem of floodplain forest degradation in the steppe zone, using the Don basin’s rivers as an example. The floodplains of the Khoper, Buzuluk, Ilovlya, Medveditsa, and Chir rivers were the key objects of the study. Identification of the landscape-hydrogeological conditions of river floodplains was carried out using the profiling method. In total, five transverse landscape profiles were laid in the floodplains of the listed rivers, along which leveling was carried out to build a hypsometric profile and a comprehensive soil and plant description. For the arid zone rivers, ground feeding is an important source of moisture in addition to atmospheric. Therefore, one of the main tasks was to determine the groundwater level using subsurface sounding (georadar). The analysis of the maximum levels of data for spring floods showed a trend of decreasing the height of floods since the late 1990s. Comparison of these data with the hypsometric profile and radar survey data revealed that the provision of complete flooding of floodplains occurs once every 10–20 years. The absence of annual complete flooding of floodplains with water has led to a decrease in the groundwater level to 5–6 m, which is an unfavorable factor for the development of woody vegetation. The climatic and hydrological conditions of the steppe zone considered in the article are unfavorable for the development of woody vegetation since they do not correspond to the conditions of its initial formation. In view of the existing conditions and the identified trends in the reduction of floods and the fall of the groundwater level, it is possible to predict a further reduction and redistribution of the forest cover of the Don basin’s river floodplains.

Contributions of terrestrial and oceanic moisture sources to orbital-scale precipitation variations over the northern East Asian monsoon region

Contributions of terrestrial and oceanic moisture sources to orbital-scale precipitation variations over the northern East Asian monsoon region

Spatial and meteorological controls of stable water isotope dynamics of precipitation in Kashmir Valley, Western Himalaya, India

ABSTRACT In the Himalayas, the lives and livelihoods of millions of people are sustained by water resources primarily depending on the moisture brought by Western Disturbances and Indian Summer Monsoon. In the present study, a network of 12 precipitation stations was established across the Kashmir Valley to understand the spatial and meteorological factors controlling precipitation isotopes. Temperature and relative humidity are dominant meteorological factors, whereas altitude, proximity to forest canopy, land use/land cover, windward and leeward sides of the mountains are the main physical factors influencing precipitation isotopes. The study suggests that the Mediterranean Sea and nearby water bodies along with continental recycling are the dominant sources of moisture from October to May, while the Arabian Sea, Bay of Bengal and continental recycling are the main sources of moisture from June to September. However, some precipitation events from October to May collect moisture from the Arabian Sea and some precipitation events from June to September collect moisture from the Mediterranean Sea. The occasional passage of Western Disturbances in summer merging with the Indian Summer Monsoon yields heavy to very heavy precipitation. The study provides a better understanding of complex spatial and meteorological phenomena controlling precipitation isotopes across the Western Himalayas.

Compositions of hydrogen and oxygen isotopes of precipitation in Xiamen, southeast China coast: Seasonal variations, synoptic processes, and typhoons impact

AbstractIn this study, the δ2H and δ18O values of 162 precipitation samples (including 33 typhoon‐related samples), collected in Xiamen, southeast China coast, during June 2018 to August 2019, were investigated and analysed. The findings indicate an obvious seasonal variation in the δ2H and δ18O, primarily influenced by the East Asia Monsoon system with notable impact from typhoon occurrences during summer months. The synoptic time‐series variation of δ2H and δ18O exhibits greater complexity: three distinct types of isotopic variations were observed, indicating a clear influence from re‐evaporation processes and continuing equilibrium fractionation during condensation. The variation patterns of δ2H and δ18O were dominated by the local meteorological parameters that govern the re‐evaporation, whereas the overall isotopic composition of precipitation was primarily regulated by the moisture sources. The unique atmospheric structure and dynamic processes of typhoons led to a three‐stage variation of δ2H and δ18O in precipitation as the distance between the typhoon's centre and the study area changed. The consistent configuration of typhoon structure prompts analogous staged changes in different typhoon‐related precipitation. The trajectory of moisture source for typhoon‐related precipitation demonstrates a clear spiral structure (excluding typhoon Yutu), revealing notable impacts from typhoon events.

Synoptic circulation patterns of urban floods for the city of Hyderabad

AbstractSynoptic fingerprinting of flood driving rainfall events over the Hyderabad city, India were assessed through an unsupervised machine learning technique—self‐organizing maps (SOM) for the period 1979–2020. Flood dates were identified using nonscientific data sources, and then large‐scale climate variables, that is, integrated vapour transport (IVT) and geopotential height at 500 hPa (Z500) along with rainfall at daily scale were considered. SOM was applied on IVT and results indicated large spatial scale‐ and season‐specific mechanisms in addition to local circulation systems—this highlights SOM's ability to cluster circulation mechanisms as per the relevance. The rainy seasons, that is, monsoon and postmonsoon showed well‐known moisture pathways. While synoptic systems can be easily linked with monsoon and postmonsoon mechanisms, it is interesting to note that the local systems exist even during the monsoon and postmonsoon season in addition to the summer season. Spatial patterns of Z500 corresponding to SOM nodes of IVT showed the presence of low‐pressure systems (LPSs), tropical cyclones and trough systems. Further analysis was performed, and a total of 35 atmospheric rivers (ARs) were identified. Plots of back trajectories for the selected AR events indicated that Arabian Sea is the main source of moisture in the monsoon season followed by Bay of Bengal for postmonsoon and tropical cyclones in summer season events. Additionally, moisture from land is also observed as another source of moisture. Back‐trajectory analysis further indicated AR‐LPS interaction during the monsoon and ARs feeding moisture to LPSs. Overall, well‐known and season‐specific synoptic systems that brought significant rainfall to the study region were identified by the SOM. The proposed framework is adaptable for different locations, and as floods in cities across the globe are on the rise, these findings will have a prominent role in urban flood forecasting and management.

Formula omitted] in speleothem carbonates in Soreq Cave as an archive for hydro-climatic conditions

δ18O in speleothem carbonates is a common archive for paleoclimate on land. Recently, it has been shown that triple oxygen isotopes in CaCO3 (given as Oexcess17=106[ln⁡(10−3δ17O+1)−0.528(ln⁡(10−3δ18O+1)]) record Oexcess17 of its parent water, thus providing additional paleo-hydrology information, primarily about relative humidity. The O17 fractionation between CaCO3 and water has been determined in biogenic and synthetic carbonates. In speleothems, however, this fractionation is expected to be modified by kinetic isotope effects associated with CO2 degassing from cave drip water. Here, we used Soreq Cave as a case study to examine this fractionation and the use of Oexcess17 in speleothems as an archive for paleo rainfall composition. We first characterized Oexcess17 in rainfall collected above the cave. The Oexcess17 value in amount weighted integrated rainfall was 49 per meg, consistent with the low relative humidity at the moisture source of the Eastern Mediterranean Sea. The measured cave water was slightly lower in Oexcess17 and higher in δ18O relative to rainfall, due to modifications of the infiltrating water by epikarst processes such as evaporation and mixing. Oexcess17 in CO2 extracted from CaCO3 of modern speleothems was compared to that in modern cave water, to calculate the carbonate-water fractionation slope (θ=ln17⁡α/ln18⁡α). The resulting θ in Soreq Cave was 0.5230±0.0002, consistent with that observed in other carbonates, indicating the lack of significant dis-equilibrium effects in speleothems Oexcess17. Hence, the common fractionation slope can be used together with a speleothem-specific α18 to reconstruct rainfall Oexcess17 from speleothem carbonates. Whereas speleothems from a variety of caves can be used through this approach to reconstruct Oexcess17 in paleo rainfall, the atmospheric processes that govern rainfall Oexcess17 are expected to be region-specific. Oexcess17 measurements in ancient Soreq speleothems are expected to reveal potential glacial-interglacial changes in moisture source conditions as well as possible changes in epikarst processes over time.

Moisture Sources of the Tohoku Heavy Rainfalls in August 2022 and the Influences of Tropical Storms

AbstractIn August 2022, the Tohoku region of Japan suffered three major heavy rainfall events, while one was related to an anticyclone, and the others were induced by a “Baiu‐like” stationary front. By using the Lagrangian backward trajectories, we found the moisture sources of the three events varied much due to the rapidly changing atmospheric conditions. In general, moisture from the Sea of Japan contributed the most in all events, while the western Pacific also played a certain role. Surprisingly, moisture from tropical storms and the East China Sea did not contribute much due to the along‐transport precipitation. But, based on forward‐traced trajectories, we confirmed that tropical storms did enhance moisture transport from subtropical regions via their southerly wind. Overall, this study highlights the role of the moisture gains nearby and the outer flows of tropical storms during heavy rainfalls in a northern region.

Sensitivity of Extreme Rainfall in Taiwan to SST Over the South China Sea Through Modulation of Marine Boundary Layer Jet: A Mei‐Yu Front Event During 1–4 June 2017

AbstractWater vapor transport from the South China Sea (SCS) by the marine boundary layer jet (MBLJ) can be an important moisture source for heavy rainfall events in Taiwan during the mei‐yu season. However, the variability in the MBLJ due to sea surface temperature (SST) changes and its impact on extreme rainfall events have yet to be well understood. This study investigates how varying the SST over SCS modifies the structure of MBLJ and further impacts an extreme mei‐yu rainfall in Taiwan during 1–4 June 2017. Results show that SST over the SCS can affect rainfall in Taiwan through its modulation of the MBLJ. Increasing SST leads to a southward shift of the mei‐yu rainfall. This is attributed to the weakening of the MBLJ and the enhanced convection at the leading edge of the mei‐yu front. The opposite effect is observed when the SST is decreased but with a smaller impact.

Characteristics of mean and extreme precipitation in Ny Ålesund, Arctic

The intensification of precipitation in the Arctic can impact the ecosystem by altering the freshwater flux, albedo, sea ice, and permafrost stability. In a location like the Arctic with a harsh environment, precipitation observations are difficult to make. In the present study, precipitation characteristics are investigated using precipitation records from Ny Ålesund, Svalbard, where the longest record of precipitation in the Atlantic sector of the Arctic is available. Precipitation is expected to increase with the warming of the atmosphere; however, significant trends in precipitation are only observed during fall (0.36 mm/decade). Even though a strong temperature increase was noticed in winter, the precipitation trend was not significant. Precipitation is separated into weak and extreme events to gain more insight into its characteristics. Again, fall was characterised by a significant positive trend in weak precipitation (150 × 10−3mm/decade), while extreme winter precipitation displayed a positive trend during 2000–2019 (4.13 mm/decade). Further analysis indicated that winter extreme precipitation followed a 2-CC relationship while the precipitation during fall followed 2-CC/1-CC scaling for mean/extreme events. The atmospheric circulation pattern during the extreme events resembles the Scandinavian pattern, characterised by high MSLP anomalies over northwestern Europe. While widespread evaporation was noticed over the North Atlantic during extreme events, vertically integrated moisture divergence indicated West Spitzbergen Current (WSC) could be a possible moisture source for extreme precipitation events during winter.

Temperature variability over the past three centuries in the coastal region of Princess Elizabeth Land, Antarctica, reconstructed from ice core stable isotope records

High-resolution stable isotope records obtained from Antarctic ice cores can be used as proxies to investigate past climatic changes in Antarctica, overcoming the spatiotemporal limitations of observational and instrumental records. Here, we used a new high-resolution ice core stable isotope record (1709–2001 AD) obtained from the Lambert Glacier Basin 69 (LGB69) site and a published stable isotope record (1757–1987 AD) from coastal Princess Elizabeth Land (PEL) to reconstruct temperature variability in the coastal region of PEL over the past three centuries. The dominant moisture source region for coastal PEL is mid-high latitudes of the South Indian Ocean (SIO) and this has remained stable over the past three centuries. Owing to non-climatic noise, no statistically significant isotope‒temperature relationship can be evidenced at annual-to-decadal scales. However, the ice core δD records do enable us to reconstruct multi-decadal temperature variability in the coastal region of PEL. The temperature reconstructions showed a consistent warming trend during the 20th century, whereas the cold periods prior to the 20th century may be related to the Little Ice Age (LIA). Our reconstructions exhibit high reliability, evidenced by their agreement with previous reconstructions from PEL. We also found that annual temperature variability is related to the Southern Annular Mode (SAM), whereas multi-decadal temperature variability is related to the Indian Ocean Dipole (IOD). However, the relationship between the SAM and temperature is unclear before the 1970s. The results of this study are helpful in understanding long-term temperature changes in Antarctica and provide an important reference for the attribution of temperature changes.

Microclimatic Variation in Miri Region (NW Borneo): Inference from Rainfall and Temperature Trends, Isotopic Signature and Air Mass Movement

Trend analysis is frequently utilised to identify the changes in meteorological and hydrologic time series data, such as rainfall and temperature. The variations in the intensity, rainfall pattern and temperature have gradually changed globally. Hence, in this study, an attempt was made to analyse the decadal rainfall and surface air temperature data to understand the microclimatic variations in the Miri coastal region of NW Borneo. A data series of records for daily total rainfall amount and daily surface temperature of 11 years from 2010 to 2021 was studied and analysed. In addition, representative rainwater and groundwater samples were collected and analysed for hydrochemical parameters and oxygen and hydrogen isotopes. A detailed literature review was carried out on rainfall patterns in Malaysia, which was used for the comparative study. Interpretation of results shows that the northeast monsoon (NEM) contributed a higher total rainfall rate with lower daily mean surface air temperature over the years compared to the southwest monsoon (SWM). The recorded data for rainfall amounts in SWM for the month of May, July, August and September were higher, particularly for the years 2010 and 2020. During NEM, a higher rainfall amount was recorded in the month of January for several years. February month has always been among the driest month in NEM, and September has been the wettest month throughout the year during SWM. The isotopic values of rainwater indicate a similar moisture source to the regional precipitation trend. Groundwater isotopes reveal the low water-rock ratio of retrograde exchange between water and primary silicate minerals in the aquifer. The moisture source of the precipitation was contributed from both oceanic and continent, affecting the rainfall intensity in this region. This study is a crucial outcome to determine the potential impacts of microclimatic variations on the rainfall patterns in the Miri coastal region.

High‐resolution in situ stable isotope measurements reveal contrasting atmospheric vapour dynamics above different urban vegetation

AbstractWe monitored stable water isotopes in liquid precipitation and atmospheric water vapour (δv) using in situ cavity ring‐down spectroscopy (CRDS) over a 2 month period in an urban green space area in Berlin, Germany. Our aim was to better understand the origins of atmospheric moisture and its link to water partitioning under contrasting urban vegetation. δv was monitored at multiple heights (0.15, 2 and 10 m) in grassland and forest plots. The isotopic composition of δv above both land uses was highly dynamic and positively correlated with that of rainfall indicating the changing sources of atmospheric moisture. Further, the isotopic composition of δv was similar across most heights of the 10 m profiles and between the two plots indicating high aerodynamic mixing. Only at the surface at ~0.15 m height above the grassland δv showed significant differences, with more enrichment in heavy isotopes indicative of evaporative fractionation especially after rainfall events. Further, disequilibrium between δv and precipitation composition was evident during and right after rainfall events with more positive values (i.e., values of vapour higher than precipitation) in summer and negative values in winter, which probably results from higher evapotranspiration and more convective precipitation events in summer. Our work showed that it is technically feasible to produce continuous, longer‐term data on δv isotope composition in urban areas from in situ monitoring using CRDS, providing new insights into water cycling and partitioning across the critical zone of an urban green space in Central Europe. Such data have the potential to better constrain the isotopic interface between the atmosphere and the land surface and to thus, improve ecohydrological models that can resolve evapotranspiration fluxes.

Monitoring rising damp in solid masonry walls: An experimental comparison of five different methods

This study focuses on the monitoring of rising damp, a widespread phenomenon that causes moisture-related damages in existing construction. Diagnosing moisture-related damages in construction can be difficult due to the many potential causes of moisture accumulation. The study aims to contribute to the understanding and diagnosis of moisture sources by evaluating and comparing different monitoring techniques for the diagnosis of rising damp in different types of masonry (brick, stone, and reinforced concrete). Five methods were evaluated and compared: gravimetric test, wall weighing, temperature and humidity sensors placed inside the wall, microwave instrumentation, and thermographic images. The results of the study showed that the gravimetric test is the only method that provides reliable and quantitative data, but it has limitations such as the need to ensure that the extracted powders consist of a single material and the potential for heat to cause moisture to evaporate quickly. The weighing method is time-consuming and limited to lab settings, and the use of sensors placed inside the wall is semi-invasive but has important limitations such as sensor reliability and different results in masonry with heterogeneous composition. The data obtained with microwave instruments cannot be easily correlated with the amount of moisture, and the method proposed for the analysis of thermographic images allows for the assessment of the evaporation area over time but has greater uncertainty in some replicas. As a result if the study, a combined approach with complementary methods is suggested to reach more accurate results.

Quantifying the Importance of Selected Drought Indicators for the United States Drought Monitor

Abstract Using information theory, our study quantifies the importance of selected indicators for the U.S. Drought Monitor (USDM) maps. We use the technique of mutual information (MI) to measure the importance of any indicator to the USDM, and because MI is derived solely from the data, our findings are independent of any model structure (conceptual, physically based, or empirical). We also compare these MIs against the drought representation effectiveness ratings in the North America Drought Indices and Indicators Assessment (NADIIA) survey for Köppen climate zones. This reveals 1) agreement between some ratings and our MI values [high for example indicators like standardized precipitation evapotranspiration index (SPEI)]; 2) some divergences (e.g., soil moisture has high ratings but near-zero MIs for ESA Climate Change Initiative (CCI) soil moisture in the Western United States, indicating the need of another remotely sensed soil moisture source); and 3) new insights into the importance of variables such as snow water equivalent (SWE) that are not included in sources like NADIIA. Further analysis of the MI results yields findings related to 1) hydrological mechanisms (summertime SWE domination during individual drought events through snowmelt into the water-scarce soil); 2) hydroclimatic types (the top pair of inputs in the Western and non-Western regions are SPEIs and soil moistures, respectively); and 3) predictability (high for the California 2012–17 event, with longer-time scale indicators dominating). Finally, the high MIs between multiple indicators jointly and the USDM indicate potentially high drought forecasting accuracies achievable using only model-based inputs, and the potential for global drought monitoring using only remotely sensed inputs, especially for locations having insufficient in situ observations. Significance Statement Drought maps from the U.S. Drought Monitor and the Objective Short- and Long-Term Drought Indicator Blends and Blend Equivalents are integrated information sources of the different types of drought. Multiple indicators go into creation of these maps, yet it is usually not clear to both public and private stakeholders like local agencies and insurance companies about the importance of any indicator in any region and season to the drought maps. Our study provides such objective information to enable understanding the mechanism and type of drought occurring at a location, season, and possibly event of interest, as well as to potentially aid in better drought monitoring and forecasting using smaller custom sets of indicators.